During the past several decades, the use of semiconductors has become ever increasingly widespread and important. Silicon based semiconductors, for example, have generally been successful in providing a variety of useful devices, such as p-n junction rectifiers (diodes), transistors, silicon control rectifiers (SCR's), photovoltaic cells, light sensitive diodes, and the like. However, due to the high cost of producing crystalline silicon and the ever-increasing demand for semiconductors over a broadening range of applications, there has been a need to widen correspondingly the scope of available useful semiconductor materials.
Useful semiconductors of the present invention, have an energy band gap in the range of about 1 to 3 eV (more specifically 1.4 to 2.2 eV); a photoconductive ratio greater than 5, (more specifically between 100 to 10,000); a conductivity between about 10.sup.-5 and 10.sup.-12 ohm-cm (more specifically conductivity in the range of 10.sup.-8 to 10.sup.-9 ohm-cm); and chemical and physical stability under ambient operating conditions. Accordingly, while many materials may be semiconducting in the sense they are not pure metals or pure insulators, only those semiconducting materials which meet these criteria may be considered to be useful semiconductors in the context of this invention.
Given the present need to develop alternative non-petroleum based energy sources, the potential commercial utility of a semiconductor increases dramatically when the semiconductor also exhibits an effective photovoltaic characteristic, that is, the ability to economically and efficiently convert solar energy into electrical potential.
From an economic standpoint, amorphous semiconductors, particularly in the form of thin films, are more desirable than single crystalline forms due to potential lower cost of production. Amorphous semiconductors also have better electrical qualities than polycrystalline forms of the same material as used in many semiconductor devices.
The semiconductor industry has continued its search for useful new semiconductor materials beyond crystalline silicon, and the like.
In the non-silicon crystalline area, single crystals of semiconducting compounds, including GaAs, GaP, and InP, are in commercial use.
Many other semiconductor materials have been utilized for specialized purposes. For example, CdS and selenium are utilized as the photoconductor in many xerographic machines.
In this application semiconductor device means a device including a semiconductor material whether the device employs electrical contacts, that is, is an electronic device, or whether it is a non-electronic device, such as the photoconductors employed in xerography, phosphorescent materials, the phosphorus in a cathode ray tube, or the like.
Although some of the known forms of phosphorus have been stated to have semiconducting properties, many are unstable, highly oxidizable and reactive, and no known form of phosphorus has been successfully employed as a useful semiconductor.
The group 3-5 materials such as gallium phosphide and indium phosphide are tetrahedrally bonded and thus, as will be pointed out below, are clearly distinguished from the compounds disclosed herein. Furthermore, their semiconducting properties are not dominated by phosphorus-to-phosphorus bonding, i.e. the primary conduction paths are not the phosphorus-to-phosphorus bonds.
Others have disclosed hydrogenated phosphorus having a structure similar to black phosphorus and having semiconducting properties.
Considerable work on high phosphorus polyphosphides has been done by a group headed by H. G. von Schnering. The various reports from this group indicate that the highest phosphorus containing polyphosphide compound they have produced is crystalline MP.sub.15 (M=group 1a metal). These polyphosphides are produced by heating a mixture of metal and phosphorus in a sealed ampule. Von Schnering reports that based on their structure polyphosphides are classified as valence compounds in a classical sense, and that this means that these compounds are, or should be, insulators or semiconductors, i.e. not metals.
Monoclinic phosphorus, also called Hittorf's phosphorus, is prepared according to the prior art from white phosphorus and lead as follows: 1 g of white phosphorus and 30 g of lead are heated slowly to a melt in a sealed tube to 630.degree. C. and held for a short time at that temperature. The solution is then cooled at the rate of 10.degree. per day for 11 days to 520.degree. C., and cooled rapidly to room temperature thereafter. It is next electrolyzed in a solution of 2 kg of lead acetate in 8 liters of 6% acetic acid, and the phosphorus is collected in a watch glass placed under the anode. Nearly square tabular crystals, about 0.2.times.0.2.times.0.05 mm, are obtained in this way.
The structure of this prior art monoclinic phosphorus has been determined by Thurn and Krebs. The crystals comprise two layers of pentagonal tubes of phosphorus with all of the tubes parallel, and then another pair of layers of all pentagonal tube phosphorus, the tubes in the second pair of layers all being parallel, but the tubes in the second pair of layers being perpendicular to the tubes in the first pair of layers. The space group of the crystals has been determined, as well as the bond angles and bond distances. See the summary of the prior art in the section "Phosphorus" from "The Structure of the Elements" by Jerry Donahue, published in 1974.
The electronic properties of Hittorf's phosphorus crystals have not been reported. Because of their small size the electrical properties cannot be readily determined.
The preparation of high purity electronic grade phosphorus according to the prior art is very complex and time consuming, thus electronic grade phosphorus is very expensive.
The prior art also exhibits a need for stable phosphorus compounds for use as fire retardants. Crystalline forms have additional utility as reinforcing additives in plastics, glasses and other materials.